1. Field of the Invention
The present invention relates to a corona discharge device and an ion mobility spectrometer having the corona discharge device.
2. Description of the Related Art
An ion mobility spectrometer achieves differentiating of ions based on the fact that different ions drift at different speeds in a uniform weak electric field. The ion mobility spectrometer has the following advantages. The ion mobility spectrometer is capable of differentiating ions quickly, has a high sensitivity, does not need a vacuum environment, and facilitates miniaturization. Therefore, the ion mobility spectrometer is widely used in the field of detection of drugs and explosives. A typical ion mobility spectrometer is generally composed of a sample feeding part, an ionization region, an ion door, a drift region, a collection region, a reading circuit, a data acquiring and processing part, a control part, and the like. The ionization part mainly functions to convert molecules of a sample into ions that can be drifted so as to be separated. Therefore, the effect of ionization affects performance of the spectrometer very directly. Among current techniques, the common and widely used ionization assembly is one which employs a Ni63 radiation source. The ionization assembly has a small volume and a high stability, and does not need any additional circuit, but brings about a narrow linear range, a lower concentration of converted ions, and radioactive contamination. Especially the radioactive contamination causes much inconvenience to the operation, transportation and management of the apparatus.
One solution for overcoming the radioactive contamination is to adopt corona discharge technology instead of radiation source technology. The corona discharge is a phenomenon in which ionization of molecules of gas is caused due to a local strong electric field in a spatial nonuniform electric field. Ions generated directly by the corona discharge are generally called reactant ions. When molecules of a sample having a higher proton or electron affinity pass through the ionization region, they capture electric charge of the reactant ions so as to be ionized. Generally, a structure for the corona discharge is relatively simple and thus has a low cost, while a concentration of electric charge generated by the corona discharge is far higher than that generated by a radiation source. Therefore, the corona discharge facilitates improvement of sensitivity of the ion mobility spectrometer and obtains a large dynamic range.
However, it has disadvantages to perform ionization by using corona discharge. The corona discharge needs a high-voltage power source for supplying electric power. Furthermore, because the corona discharge itself occurs in a pulsed process (a Trichel pulse), a disorder of spectrum lines will be caused such that a detection result will be seriously affected if ions are allowed to enter the ion mobility spectrometer directly through the ion door. In addition, ions in a region of the corona discharge will be accelerated by an electric field in the region to strike a corona electrode so as to be lost. As a result, improvement of the sensitivity of the ion mobility spectrometer is inhibited. How to effectively drag out the ions from the ionization region is still a serious problem to be solved. In addition, since the corona discharge will causes oxidation of the corona electrode, it is an important problem to lengthen a life time of the electrode.